The digitization of voice samples in time-division-multiplex (TDM) telecommunication systems is well known. Thus, a code word of 12 bits -- including a polarity-indicating sign bit -- can be used to express 2.sup.11 = 2048 distinct amplitude levels of a bipolar signal divided into eight ranges which are of identical widths on a logarithmic scale, i.e. a lowest range of 0- 15 units (e.g. millivolts), a second range of 16- 31 units, a third range of 32- 63 units etc., up to a highest range of 1024- 2047 units. As taught in commonly owned U.S. Pat. Nos. 3,755,808 and 3,789,392 issued in the name of Giampiero Candiani, such ordinary digital code words can be compressed into eight-bit codes and then re-expanded into their original 12 bits, with only minor loss of information, by expressing the rank of each code -- i.e. the order number of its amplitude range -- in binary form by means of three range bits which immediately follow the sign bit and are in turn succeeded by four quantum bits X, Y, Z, W representing the most significant bits following directly after the first "one" (or after the eighth "zero" in the case of the lowermost range) in the original code word. In more general terms, such a compressed code word consists in descending order of a sign or polarity bit, m range bits and n quantum bits.
A peculiarity of this type of compressed code word is the fact that the numerical weight of its quantum bits doubles from the second-lowest to the highest range while being the same in the two lowermost ranges. This prevents the direct positive or negative algebraic summing (i.e. adding or subtracting) of the range bits of two such codes, derived from a pair of coincident voice samples, unless they happen to be either of the same rank or of the two lowest ranks, referred to hereinafter as rank 0 and rank 1. Such algebraic summation is necessary in a conference call in which each participating subscriber is to receive superposed voice samples from all the other participants. It has therefore been the practice, in an exchange serving PCM channels of this type engaged in a conference call, to expand the incoming compressed codes for linear superposition and to recompress the resulting codes for transmission to PCM terminals serving the receiving subscribers. Such double conversion requires complex and correspondingly expensive central-office equipment.